Constant temperature anemometers or CTAs are known to the prior art. In such devices, a heated resistance element serves as a sensing element in a wind tunnel. The sensing element has a temperature coefficient of resistance and is maintained at constant resistance and, thus, temperature.
Typical prior art constant temperature anemometers place the sensor in one leg of a bridge circuit with a feedback circuit, including an amplifier, being employed to maintain balance on the bridge. An example of such an anemometer is illustrated in FIG. 1, wherein the sensing element or sensor is designated at 10 and forms one leg of a bridge, the other bridge legs being formed by resistances 11 through 13. An operational amplifier 14 has its inputs and outputs interconnected with the bridge.
The bridge leg of sensor 10 may be considered to be serially connected with the leg of resistance 11 with the junction between them being connected to the inverting input of the amplifier 14. Similarly, the leg of resistance 12 may be considered as serially connected with the leg of resistance 13 with the junction between them being connected to the non-inverting input to operational amplifier 14. The serially connected sensor 10 and resistance 11 are connected in parallel with the serially connected resistances 12 and 13, with the junction between the leg of resistance 11 and the leg of resistance 12 being connected to the output of the operational amplifier 14. The junction between the leg of sensor 10 and the leg of resistance 13 is connected to ground.